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[2003:cb:c708:f600:abad:360:c840:33fa]) by smtp.gmail.com with ESMTPSA id l25-20020a05600c1d1900b003a62052053csm10503423wms.18.2022.08.26.07.47.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 26 Aug 2022 07:47:23 -0700 (PDT) Message-ID: <72146725-3d70-0427-50d4-165283a5a85d@redhat.com> Date: Fri, 26 Aug 2022 16:47:22 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.12.0 Subject: Re: [PATCH v3 2/3] mm/migrate_device.c: Copy pte dirty bit to page To: Peter Xu , Alistair Popple References: <3b01af093515ce2960ac39bb16ff77473150d179.1661309831.git-series.apopple@nvidia.com> <8735dkeyyg.fsf@nvdebian.thelocal> <8735dj7qwb.fsf@nvdebian.thelocal> From: David Hildenbrand Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "Sierra Guiza, Alejandro \(Alex\)" , Huang Ying , Ralph Campbell , Lyude Paul , Karol Herbst , Nadav Amit , Felix Kuehling , linuxppc-dev@lists.ozlabs.org, LKML , Matthew Wilcox , linux-mm@kvack.org, Logan Gunthorpe , Ben Skeggs , Jason Gunthorpe , John Hubbard , stable@vger.kernel.org, akpm@linux-foundation.org, huang ying Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 26.08.22 16:32, Peter Xu wrote: > On Fri, Aug 26, 2022 at 11:02:58AM +1000, Alistair Popple wrote: >> >> Peter Xu writes: >> >>> On Fri, Aug 26, 2022 at 08:21:44AM +1000, Alistair Popple wrote: >>>> >>>> Peter Xu writes: >>>> >>>>> On Wed, Aug 24, 2022 at 01:03:38PM +1000, Alistair Popple wrote: >>>>>> migrate_vma_setup() has a fast path in migrate_vma_collect_pmd() that >>>>>> installs migration entries directly if it can lock the migrating page. >>>>>> When removing a dirty pte the dirty bit is supposed to be carried over >>>>>> to the underlying page to prevent it being lost. >>>>>> >>>>>> Currently migrate_vma_*() can only be used for private anonymous >>>>>> mappings. That means loss of the dirty bit usually doesn't result in >>>>>> data loss because these pages are typically not file-backed. However >>>>>> pages may be backed by swap storage which can result in data loss if an >>>>>> attempt is made to migrate a dirty page that doesn't yet have the >>>>>> PageDirty flag set. >>>>>> >>>>>> In this case migration will fail due to unexpected references but the >>>>>> dirty pte bit will be lost. If the page is subsequently reclaimed data >>>>>> won't be written back to swap storage as it is considered uptodate, >>>>>> resulting in data loss if the page is subsequently accessed. >>>>>> >>>>>> Prevent this by copying the dirty bit to the page when removing the pte >>>>>> to match what try_to_migrate_one() does. >>>>>> >>>>>> Signed-off-by: Alistair Popple >>>>>> Acked-by: Peter Xu >>>>>> Reported-by: Huang Ying >>>>>> Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages") >>>>>> Cc: stable@vger.kernel.org >>>>>> >>>>>> --- >>>>>> >>>>>> Changes for v3: >>>>>> >>>>>> - Defer TLB flushing >>>>>> - Split a TLB flushing fix into a separate change. >>>>>> >>>>>> Changes for v2: >>>>>> >>>>>> - Fixed up Reported-by tag. >>>>>> - Added Peter's Acked-by. >>>>>> - Atomically read and clear the pte to prevent the dirty bit getting >>>>>> set after reading it. >>>>>> - Added fixes tag >>>>>> --- >>>>>> mm/migrate_device.c | 9 +++++++-- >>>>>> 1 file changed, 7 insertions(+), 2 deletions(-) >>>>>> >>>>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c >>>>>> index 6a5ef9f..51d9afa 100644 >>>>>> --- a/mm/migrate_device.c >>>>>> +++ b/mm/migrate_device.c >>>>>> @@ -7,6 +7,7 @@ >>>>>> #include >>>>>> #include >>>>>> #include >>>>>> +#include >>>>>> #include >>>>>> #include >>>>>> #include >>>>>> @@ -196,7 +197,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, >>>>>> anon_exclusive = PageAnon(page) && PageAnonExclusive(page); >>>>>> if (anon_exclusive) { >>>>>> flush_cache_page(vma, addr, pte_pfn(*ptep)); >>>>>> - ptep_clear_flush(vma, addr, ptep); >>>>>> + pte = ptep_clear_flush(vma, addr, ptep); >>>>>> >>>>>> if (page_try_share_anon_rmap(page)) { >>>>>> set_pte_at(mm, addr, ptep, pte); >>>>>> @@ -206,11 +207,15 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, >>>>>> goto next; >>>>>> } >>>>>> } else { >>>>>> - ptep_get_and_clear(mm, addr, ptep); >>>>>> + pte = ptep_get_and_clear(mm, addr, ptep); >>>>>> } >>>>> >>>>> I remember that in v2 both flush_cache_page() and ptep_get_and_clear() are >>>>> moved above the condition check so they're called unconditionally. Could >>>>> you explain the rational on why it's changed back (since I think v2 was the >>>>> correct approach)? >>>> >>>> Mainly because I agree with your original comments, that it would be >>>> better to keep the batching of TLB flushing if possible. After the >>>> discussion I don't think there is any issues with HW pte dirty bits >>>> here. There are already other cases where HW needs to get that right >>>> anyway (eg. zap_pte_range). >>> >>> Yes tlb batching was kept, thanks for doing that way. Though if only apply >>> patch 1 we'll have both ptep_clear_flush() and batched flush which seems to >>> be redundant. >>> >>>> >>>>> The other question is if we want to split the patch, would it be better to >>>>> move the tlb changes to patch 1, and leave the dirty bit fix in patch 2? >>>> >>>> Isn't that already the case? Patch 1 moves the TLB flush before the PTL >>>> as suggested, patch 2 atomically copies the dirty bit without changing >>>> any TLB flushing. >>> >>> IMHO it's cleaner to have patch 1 fix batch flush, replace >>> ptep_clear_flush() with ptep_get_and_clear() and update pte properly. >> >> Which ptep_clear_flush() are you referring to? This one? >> >> if (anon_exclusive) { >> flush_cache_page(vma, addr, pte_pfn(*ptep)); >> ptep_clear_flush(vma, addr, ptep); > > Correct. > >> >> My understanding is that we need to do a flush for anon_exclusive. > > To me anon exclusive only shows this mm exclusively owns this page. I > didn't quickly figure out why that requires different handling on tlb > flushs. Did I perhaps miss something? GUP-fast is the magic bit, we have to make sure that we won't see new GUP pins, thus the TLB flush. include/linux/mm.h:gup_must_unshare() contains documentation. Without GUP-fast, some things would be significantly easier to handle. -- Thanks, David / dhildenb